Avoid nil pointer exception if attempt is made to join when there is no nwkmgr connection.
Avoid nil pointer exception if attempt is made to join when there is no nwkmgr connection.

ref: https://bugsnag.com/ninja-blocks-1/driver-go-zigbee/errors/54ae7288c3bb24b16470251a

Signed-off-by: Jon Seymour <44f878afe53efc66b76772bd845eb65944ed8232@ninjablocks.com>

ninjasphere/driver-go-zigbee,ninjasphere/driver-go-zigbee